Materials for chromatographic separations can be generally classified as inorganic (e.g., silica), organic (e.g., polydivinylbenzene), or hybrid inorganic/organic materials.
As stationary phases for HPLC, organic-based materials are chemically stable against strongly alkaline and strongly acidic mobile phases, allowing flexibility in the choice of mobile phase pH. However, organic chromatographic materials generally result in columns with low efficiency, leading to inadequate separation performance, particularly with low molecular-weight analytes. Furthermore, many organic chromatographic materials shrink and swell when the composition of the mobile phase is changed. In addition, most organic chromatographic materials do not have the mechanical strength of typical chromatographic silica.
Silica-based materials are mechanically strong and result in columns that do not show evidence of shrinking or swelling. However, limited hydrolytic stability is a drawback with silica-based columns, because silica may be readily dissolved under alkaline conditions, generally pH >8.0, leading to the subsequent collapse of the chromatographic bed. Additionally, the bonded phase on a silica surface may be removed from the surface under acidic conditions, generally pH <2.0, and eluted off the column by the mobile phase, causing loss of analyte retention.
Porous inorganic/organic hybrid materials have been introduced to overcome the above-mentioned deficiencies while attempting to maintain the beneficial properties of purely organic and purely inorganic materials. These materials are synthesized from a mixture of inorganic and organofunctional silane monomers to prepare a copolymer, e.g. SiO2/RSiO1.5 or SiO2/R(SiO1.5)2 and can be either particulate or monolithic in form. See, e.g., K. Unger, J. Schick-Kalb, U.S. Pat. No. 4,017,528; A. Sayari, S. Hamoudi, Chem. Mater. 13 (2001) 3151; K. Nakanishi, N. Soga, T. Minakuchi, U.S. Pat. No. 6,207,098; K. Nakanishi, N. Soga, Japanese patent application 2,893,104.
However, there is a number of problems with these materials when used in chromatographic separations. By and large, these problems arise because the particulate forms have been made by direct co-condensation of the monomers into a silicate form or via an intermediate poly(organosiloxane) (POS). The resultant particles are commonly irregular in shape, are not highly spherical, or have an irregular surface morphology. Because of these irregularities in shape or morphology, these particles do not afford the packing of highly efficient columns that are required for good chromatography.
The particles further contain a large population of micropores with a diameter of about <40 Å. It is known that the diffusion of a molecule in the pores of a material slows down measurably as the pore size becomes smaller than about 10 times the size of the analyte molecule, resulting in poor peak shape and band broadening. As a result, materials with a large population of micropores are not particularly useful for most chromatographic separations and have little utility.
In the case of monolithic materials, many of the monoliths lack macropores that are required for low operating backpressures. In monolith cases where macropores have been achieved, the monoliths contain a large population of micropores with a diameter of about <40 Å and suffer the same disadvantages as described above. Hybrid materials containing only a small population of micropores and a sufficient population of mesopores have been reported to solve this problem. See, e.g., Z. Jiang, R. Fisk, J. O'Gara, T. Walter, K. Wyndham U.S. Pat. No. 6,686,035, and T. Walter, J. Ding, M. Kele, J. O'Gara, P. Iraneta WO 03/014450.
However, the removal of the deleterious micropores is achieved by a hydrothermal treatment and comes at the expense of decreasing surface area, which consequently diminishes the material's retention capacity. In addition, the removal of the deleterious micropores results in a unimodal mesopore population that is polydisperse. Finally, all of the hybrid materials containing only a small population of micropores and a sufficient population of mesopores are amorphous or disordered.
Thus, there is a need for hybrid materials having ordered domains, in which chromatographically desirable morphologies (e.g., spherical particles and monoliths with a bimodal pore size distribution of macropores and mesopores) are preserved. Although the analogous preservation of purely inorganic silica gel particles has been reported, see, e.g., T. Martin, A. Galarneau, F. Di Renzo, F. Fajula, D. Plee, Angew. Chem. Int. Ed. 41 (2002) 2590, this has not yet been achieved for hybrid materials, especially those with a chromatographically-enhancing pore geometry. Therefore, porous inorganic/organic hybrid materials comprising ordered domains and, advantageously, a chromatographically-enhancing pore geometry are needed.